This invention relates to an image display system for a scanning electron microscope, and particularly to an image display system for a stroboscopic scanning electron microscope.
The stroboscopic scanning electron microscope is a scanning electron microscope with a pulse gate and a synchronous circuit added. FIG. 1 shows the basic construction of the stroboscopic scanning electron microscope (refer to G. S. Plows and W. C. Nixon; "Stroboscopic Scanning Electron Microscopy", J. Phys. E., Ser. II, pp 595-600, 1968), and FIGS. 2A and 2B are explanatory diagrams useful for explaining the principle of that microscope. Referring to FIG. 1, an electron beam 2 emitted from an electron gun 1 is focused on a sample 10 to be observed by an electron lens 6 and deflected by a scanning coil or deflector 8 as in the television camera tube. Since the electron beam, when impinging on a solid, is reflected or ejects secondary electrons therefrom, the reflected electrons or secondary electrons are detected by a detector 9, the image from which is displayed on a display 7. The principle of the scanning electron microscope has been described above.
When a sample changing at very high speed is observed by this scanning electron microscope, the scanning and/or the detecting cannot follow the change of the sample, with the result that the total change is displayed in a superimposed manner. For this reasons, a pulse gate (the combination of a deflection plate 3 and an aperture 4) is added which performs chopping of the beam in cooperation with a pulse circuit 12 synchronized with a driver 11 (provided within or out of the sample) which is changing the state of the sample. This construction can control the electron beam irradiating the sample only at a certain phase of the change of the sample, thereby to detect the condition of the sample only at the instant of irradiating the beam.
FIG. 2A is a diagram for explaining this controlling. To better understand this, it is assumed that a point of object is moving at high speed and periodically as A.fwdarw.B.fwdarw.C.fwdarw.B.fwdarw.A within the sample. If the phase of the movement is adjusted by a phase adjuster 5 so that the electron beam is emitted at time a, since the point object is always at position A at time a, it is observed still at position A as shown in FIG. 2B. Similarly, if the phase of the movement is adjusted to point b and c in turn, the point object appears still at B and C. This is the principle under which the high speed object is observed on the stroboscopic scanning electron microscope.
The stroboscopic scanning electron microscope is used mainly for observing the voltage changing at high speed within an LSI. Most of LSI's are covered by a passivation layer. This passivation layer is necessary for the following reasons. That is, when a reverse voltage is applied to a PN junction, a depletion layer which has a strong electric field is caused on the surface. Since this field is subject to the external influence, and the electric characteristics of the PN junction are determined by the parallel connection of the internal bulk and the surface layer, the electric characteristics of the PN junction have a great dependence on the characteristic of the surface layer sensitive to the external conditions and thus are unstable. Therefore, the surface of LSI must be covered by an insulating material (chiefly SiO.sub.2), which covered layer is called the passivation layer.
When the LSI covered with a passivation layer is observed on the stroboscopic scanning electron microscope, the low-energy electron beam accelerated by about 1000 V is used to avoid the passivation layer from being electrified by electron beam. In this case, the passivation layer acts as a capacitor because it is an insulating material and is interposed between the electrode to be measured and the electron beam. The presence of this capacitor means that the steady voltage (DC voltage) cannot be observed. Since the stroboscopic system makes the high speed phenomenon steady (i.e. the pulsed electron beam is irradiated on the sample to fix and observe the state of the sample), this situation is the same. Thus, the image of a constant-phase voltage within the LSI covered with the passivation layer could not be fully observed by the stroboscopic scanning electron microscope because the contrast of potential was lost.